Composite chill cast iron rolling mill rolls having increased resistance to the spalling

ABSTRACT

A COMPOSITE CHILL CAST IRON ROLL IN WHICH THE CHILL AREA HAS A COMPOSITION OF ABOUT 3.20% TO ABOUT 3.40% CAR1.45% TO ABOUT 1.65% SILICON, ABOUT 4.00% TO ABOUT 4.40% NICKEL, ABOUT 0.90% TO ABOUT 1.10% CHROMIUM, ABOUT 0.50% TO ABOUT 0.70% MOLYBDENUM AND ABOUT 0.03% TO ABOUT 0.08% MAGNESIUM, ABOUT 0.07% PHOSPHORUS, ABOUT 0.02% SULFUR, THE REMAINDER IRON AND INCIDENTAL IMPURITIES AND THE CORE IS A LOW ALLOY CAST IRON COMPOSITION OF ABOUT 3.40% TO ABOUT 3.50% CARBON, ABOUT 0.50% TO ABOUT 0.60% MANGANESE, ABOUT 1.25% TO ABOUT 1.35% SILICON, ABOUT 0.30% TO ABOUT 0.50% CHROMIUM, ABOUT 0.75% TO ABOUT 1.25% NICKEL, ABOUT 0.15% PHOSPHORUS, ABOUT 0.10% SULFUR, THE REMAINDER IRON AND INCIDENTAL IMPURITIES.

United States Patent US. Cl. 29-1961 4 Claims ABSTRACT OF THE DISCLOSUREA composite chill cast iron rollin which the chill area has acomposition of about 3.20% to about 3.40% carbon, about 0.55% to about0.65% manganese, about 1.45% to about 1.65% silicon, about 4.00% toabout 4.40% nickel, about 0.90% to about 1.10% chromium, about 0.50% toabout 0.70% molybdenum and about 0.03% to about 0.08% magnesium, about0.07% phosphorus, about 0.02% sulfur, the remainder iron and incidentalimpurities and the core is a low alloy cast iron composition of about3.40% to about 3.50% carbon, about 0.50% to about 0.60% manganese, about1.25% to about 1.35% silicon, about 0.30% to about 0.50% chromium, about0.75% to about 1.25% nickel, about 0.15% phosphorus, about 0.10% sulfur,the remainder iron and incidental impurities.

BACKGROUND OF THE INVENTION Composite chill cast iron rolls have foundlimited use as work rolls in severe applications, for example in thelatter strands of cold rolling mills. While the rolls have goodresistance to Wear and high hardness on the surface they are brittle andtherefore susceptible to spalling and have insufiicient toughness toresist bruising when contacted by the stock being worked. The stock ismarked causing it to be rejected and decreasing the product yield. Thebruise is many times a focal point for the beginning of a spall. Therolls must be removed from the mill and the surface reground to belowthe bruised area, thereby decreasing the life expectancy of the rolls.

Cold rolling blackplate requires rolls which will take a highly polishedsurface finish and which have high surface hardness, good resistance tospelling, good resistance to bruising, suflicient strength to resistdeflection stresses and which will not mark the sheet being processed.

Cold rolling sheet for exterior automotive body parts requires rollswhich, in addition to the above mentioned qualities, have a surfacewhich will take and retain a blast pattern having a finish of about 55microinches to about 70 microinches. This finish is necessary to imparta finish of about 40 microinches to about 60 microinches on the surfacesof the sheet being rolled. Heretofore the above mentioned operationsrequired the use of expensive forged steel rolls which requiredelaborate heat treatments to obtain the desired properties. However, therolls are susceptible to marking and spalling and have relatively shortservice life. The rolls of this invention resist marking and bruisingand, when required, clean up with less metal removal than the other oldprior art rolls.

It is an object of this invention to provide composite chill cast ironrolls having a reduced tendency to surface marking and spalling.

It is a further object of this invention to provide composite chill castiron rolls which may be reconditioned with comparatively light dressing.

It is a further object of this invention to produce composite chill castiron rolls which will have good resistance to wear, high hardness,sutficient strength to resist deflection stresses and increasedresistance to spalling.

Patented Nov. 30, 1971 The composite chill cast iron working rolls ofthe invention contain about 3.20% to 3.40% carbon, about 0.55% to 0.65%manganese, about 1.45% to about 1.65% silicon, about 4.00% to about4.40% nickel, about 0.90% to about 1.10% chromium, about 0.50% to about0.70% molybdenum, about 0.03% to about 0.08% magnesium, not more than.07% phosphorus and not more than 0.02% sulfur and the remainder ironand incidental impurities. Rolls having a composition within the abovespecified ranges have good resistance to wear, high surface hardness,improved resistance to bruising, increased resistance to spalling andsuflicient strength to resist deflection stresses and will not mark thematerial being rolled. By a composite chill cast iron roll I mean a rollhaving a chill portion and a core, said chill portion having acomposition as heretofore described and said core being a low alloy castiron composition as is well known in the art. Such a composition maycontain about 3.40% to about 3.50% carbon, about 0.50% to about 0.60%manganese, about 1.25% to about 1.35% silicon, about 0.30% to about0.50% chromium, about 0.75% to about 1.25% nickel, about 0.15%phosphorus, not more than 0.10% sulfur, the remainder iron andincidental impurities.

The composition of the rolls must be balanced to obtain the optimummechanical properties. The carbon content in the surface of the rollsmust be suflicient to impart the hardness necessary for the surface toresist deformation when rolling cold sheet products. However the carboncontent must be low enough to provide sufficient depth of the chill inthe roll. Therefore, I use a carbon content of between about 3.20% toabout 3.40%, but I prefer a carbon content of between about 3.25% and3.35%. The manganese must be sufficient to prevent mottling of the ironin heavy sections, that is, about 0.55% to about 0.65%. Nickel in theiron suppresses pearlite formation and favors formation of martensiteand also aids in refining carbides. The nickel should be from about4.00% to about 4.40%, but I prefer to limit the the upper range of thenickel to 4.20%. Chromium within a range of about 0.90% to about 1.10%will be suflicie-nt to stabilize the carbides and to suppress graphiteformation. However I prefer to use a range of 0.95% to about 1.05%.Molybdenum in the range of about 0.50% to about 0.70% will increase theresistance of the chill surface to spalling, however, I prefer to use arange of 0.55% to about 0.65%. While silicon aids in graphite formation,and increases the strength, the ductility and the bend test energy tofracture, too high a silicon will increase the amount of graphite formedand decrease the depth of chill. I, therefore, control the siliconcontent Within a range of about 1.45 to about 1.65%, however, I preferto use a range of 1.50% to about 1.60%. Magnesium is added to the chillcast iron to promote the formation of nodular graphite. For this purposeI prefer to control the magnesium content within a range of about 0.03%to about 0.08% and prefer a magnesium content of 0.05% to 0.07%. Theremay be a distinct line of demarcation between the chill and core areasin composite rolls, because of the sudden transition from nodular toflake graphite. In the past, this has been of great concern since thisline is a metallurgical notch and may become a focal point for spalling.It has been found to be of no consequence in the rolls of the inventionsince the transition of graphite from nodular to flake has been found tobe gradual through this area because of the comparatively low sulfurcontent of the chill area and core of the rolls.

3 The above mentioned microstructure may be obtained by subjecting therolls to a stress-relief treatment. Two such treatments are listedbelow:

Cast molten iron and cool to about 100 F., Stress relieve at 500 F.,

Hold 1 hour/ inch of thickness,

Cool to ambient temperature.

Rolls treated by the above methods were found to have a surface hardnessof Rockwell C 57.0 to 59.0, an ultimate bend strength of about 86,000 to90,000 p.s.i., a total deflection on bend test of about 0.040 to 0.045inch and a bend to fracture strength of about 1.10 ft. lbs. to about1.15 ft. lbs. and about 11.3% retained austenite in the microstructure.The bend test used to determine the ultimate bend strength, totaldeflection and toughness (bend to fracture strength) is described inInternational Nickel Company, Inc., Technical Paper 541-CP dated Nov. 3,1967, by F. K. Kies and R. D. Schelleng.

Rolls having a standard white cast iron composition and treated as abovehad a surface hardness of Rockwell C 54.0 to 56.5, an ultimate bendstrength of 77,000 to 78,000 p.s.i., a total deflection on bend test of0.037 to 0.040 inch and a bend to fracture strength of .92 to 1.03 ft.lbs., and 18% retained austenite in the microstructure of massivecontinuous carbide network, martensite, nodule graphite and eutecticaustenite. It must be understood that where percent retained austeniteis noted such percent is by volume.

EXAMPLE 1 Percent 0 Mn P S Si Ni Cr Mo Mg Chill 3. 29 0. 56 0. 06 0.012 1. 54 4. 10 0. 98 0. 61 0. 07 Core 3. 44 0. 48 0. 09 0. 04 1. 33 1.22 O. 54

The casting was cooled in the mold to about 300 F., shaken from the moldand cooled to ambient temperature. The roll was placed in a furnace andheated to 850 F. held for 4 hours, cooled at 50 F./hour to 100 F.,reheated to 500 F., held for 4 hours and cooled to ambient temperature.Test specimens from the casting had a surface hardness of Rockwell C 58,an ultimate bend strength of 88,410 p.s.i., a total deflection on bendtest of 0.042 inch and a bend to fracture strength of 1.15 ft. lbs.Microscopic examination of the test specimens showed the roll to havefinely-divided well dispersed nodules of primary graphite, fine primaryand eutectic martensite, fine austenite-martensite grains, a secondaryprecipitation of fine carbides in areas of former austenitic grains, adiscontinuous carbide network and 11.3% retained austenite.

The roll was mated with a standard white cast iron roll in the No. 5stand of a 5 stand, 4 high, 48" cold mill rolling tinplate. The pair ofrolls rolled 5,252 tons of tinplate. The standard roll was bruised fourtimes and had to be redressed to prevent marking the sheet. The roll ofthe invention was also redressed so as to mate with the standard rollalthough it had not shown evidence of bruising nor did it mark thesheet.

EXAMPLE 2 In another specific example of the invention, 2 compositetats-Q05? Chill cast iron work rolls 21 x 78" were 75 4 processed in aconventional manner. The chill and core were found to have the followingchemical compositions:

Percent 0 Mn P S Si Ni Cr Mo Mg Chill 3. 34 0.47 0. 06 0.011 1.60 4.151.01 0. 64 0.07 Core 3. 42 0.54 0. 023 1. 26 0.92 0.41

The rolls were processed in the manner described in Example 1 above andwere found to have a similar microstructure as the roll in Example 1.The rolls were ground to a finish of 20 microinches and shot-blasted toa finish of 75 microinches. The rolls were placed in a 78 inch 4 hightemper mill and were used to process sheet for use in forming exteriorautomotive body parts. The rolls processed 8 coils of sheet beforeredressing was required.

Rolls of the invention may be used as work rolls in the final stands ofa tandem mill to cold roll blackplate and in a temper mill to cold rollsheet from which exterior automotive body parts are formed. Rolls of theinvention when used in this application have processed as many as 9coils of sheet before requiring redressing due to wear of the blastpattern, while standard forged rolls process about 3 to 4 coils beforerequiring dressing. The rolls of the invention may be redressed byremoving less metal than rolls heretofore used in these applications andtherefore have a longer life expectancy than prior art rolls.

In this specification and claims wherever percentages are referred tosuch percentages are by weight unless otherwise noted.

I claim:

1. A composite work roll for cold rolling sheet stock, said roll havinga low alloy cast iron core consisting of:

Carbon: about 3.40% to about 3.50% Manganese: about 0.50% to about 0.60%Phosphorus: not more than about 0.15% ulfur: not more than about 0.10%Silicon: about 1.25% to about 1.35% Nickel: about 0.75 to about 1.25%Chromium: about 0.30% to about 0.50%

remainder iron and incidental impurities and a chill surface areaconsisting essentially of:

Carbon: about 3.20% to about 3.40% Manganese: about 0.55% to about 0.65%Phosphorus: not more than 0.07%

Sulfur: not more than 0.02%

Silicon: about 1.45% to about 1.65% Nickel: about 4.00% to about 4.40%Chromium: about 0.90% to about 1.10% Molybdenum: about 0.50% to about0.70% Magnesium: about 0.03 to about 0.08%

the remainder iron and incidental impurities, the microstructure in saidchill area comprising finely divided, welldispersed nodules of graphite,finer than normal primary and eutectic martensite andmartensite-austenite grains not more than 15% retained austenite, asecondary precipitation of carbides in areas of former austeniticgrains, and a discontinuous carbide network and characterized by havingan ultimate bend strength of about 86,000 psi. to about 90,000 p.s.i.

2. The composite work roll of claim 1 having a gritblasted surface of aroughness of between 55 microinches to about microinches.

3. A composite work roll for cold rolling sheet stock, said roll havinga low alloy cast iron core consisting of:

Carbon: about 3.40% to about 3.50% Manganese: about 0.50% to about 0.60%Phosphorus: not more than about 0.15% Sulfur: not more than about 0.10%Silicon: about 1.25% to about 1.35% Nickel: about 0.75% to about 1.25%Chromium: about 0.30% to about 0.50%

remainder iron and incidental impurities and a chill sur- 4. Thecomposite work roll of claim 3 having a gritface area consistingessentially of: blasted surface of a roughness of between 55 microinchesCarbon: about 3.25% to about 3.35% to about 70 microinches' Manganese:about 0.55% to about 0.65%

Phosphorus: not more than 0.07% 5 References Cited Sulfur: not more than0.02% UNITED STATES PATENTS Silicon: about 1.50% about 1.60% 1,910,034/1933 Mitchell et al 75128 X Nickel: about 4.00% to a o t 1,988,9101/1935 Merica et a1. 75128 X Chromium: about 0.95% to about 1.05%2,097,709 11/1937 Walters 75-128 X Molybdenum: about 0.55% to about0.65% 2,105,968 1/1938 Castle 7s 128 Magnesium: about 0.04% to about0.08% 3, 71 1 1944 d k 148--35 X the remainder iron and incidentalimpurities, the micro- 2,516,524 7/1950 Minis- 2,771,358 11/1956 Spear14835 X structure in said chill area comprising finely divided,welldispersed nodules of graphite, finer than normal primary 15 andeutectic martensite and martensite-austenite grains, not more than 15%retained austenite, a secondary pre- CHARLES LOVELL Pnmary Examinercipitation of carbides in areas of former austenitic grains, U S Cl X Rand a discontinuous carbide network and characterized by having anultimate bend strength of about 86,000 p.s.i. 75123, 128; 148-34,

to about 90,000 p.s.i.

3,273,998 9/1966 Knoth.

